Self-aligned multiple patterning, such as self-aligned double patterning (SADP), self-aligned quadruple patterning (SAQP) and direct self-assembly (DSA), are a class of technologies that are typically used to print very dense lines, such as lines having 80 nanometers (nm) pitch, 40 nm pitch or less, during the fabrication of ultra-high density semiconductor integrated circuits. These technologies generally pattern a sea (or large array) of parallel metal lines into back-end-of-line (BEOL) dielectric layers during the formation of interconnect systems for front-end-of-line (FEOL) semiconductor devices, such as transistors, resistors, capacitors or the like. The semiconductor devices are formed in an FEOL substrate layer of the semiconductor integrated circuit.
Cut masks are then utilized in a photolithographic process to pattern signal cuts into predetermined locations (or targets) of the sea of metal lines to define the tips of active metal lines and dummy metal lines. The active lines are used to route electric signals, or electric power, to and from the semiconductor devices.
The dummy lines are inactive and do not carry any signals or electric power. The dummy lines are patterned into the BEOL dielectric layer because the self-aligned multiple patterning techniques cannot distinguish between dummy and active lines. Additionally, it would become very expensive to develop a complex mask to pattern exclusively active lines.
However, due to the high density of the metal lines and the small tip to tip distances (widths), e.g. 30 nm or less, between the signal cuts, it is a significant challenge to control the cut mask's edges to prevent undercuts or unwanted cuts of a signal cut target on a specific section of a metal line. An undercut is where a targeted line may only be partially cut. An unwanted cut is where lines adjacent to the targeted line may inadvertently be cut. Both the undercuts and unwanted cuts will adversely affect system performance.
Further, a light source passing through a cut mask during a photolithographic process to illuminate a target is always subject to a certain magnitude of process errors such as focus and exposure errors that cause distortion in the images that are used to form the signal cuts. The processed images often appear with irregularities such as line widths that are narrower or wider than the design of the target image.
This distortion problem is significantly exacerbated when the signal cuts form a sparsely distributed pattern, as opposed to a densely distributed pattern, within the highly dense sea of metal lines. This is because it is virtually impossible to optimize illumination conditions in a photolithographic process for both dense lines and sparse cut patterns. Therefore, in a sparse pattern of signal cuts, the variations in the critical dimensions and other measurable features of the cuts may be three times or more than that of a dense pattern of signal cuts.
For purposes herein, a dense pattern of signal cuts is where the spaces between the cuts will approach the width of the cuts themselves. For example, in a dense pattern of signal cuts the spaces between the cuts may be less than three times the width of the cuts and preferably less than two times the width of the cuts. Also for purposes herein, in a sparse pattern of signal cuts the spaces between the cuts may be greater than three, five or more times the width of the cuts. Also, by comparison, the density of signal cuts (i.e., the number of signal cuts per unit area) for a dense pattern of cuts will generally be at least two times the density of a sparse pattern of signal cuts.
Optical proximity correction (OPC) technology can be used to reduce the above described image variations and errors by moving edges or adding features, such as sub-resolution assist features, to the pattern written into the cut masks used during the photolithographic process. However, even with the most sophisticated OPC techniques, the variations in a sparse signal cut pattern will still be on the order of three times greater than that of a dense signal cut pattern for the same size cuts.
Accordingly, there is a need for an apparatus and method to improve the resolution of metal signal cuts in active lines of a semiconductor structure. More specifically, there is a need to improve the resolution and variation of signal cuts in a sparse pattern of signal cuts. Further there is a need to improve the resolution of signal cuts which define the tip ends of active metal lines in a sea of metal active and dummy lines, wherein the metal lines have a pitch of 80 nm or less.